Narrowing Of Optical Band Gap Research Articles

Variation of structure and band gap for N doped Cu2O films deposited with ceramic target

With ceramic target, N doped Cu2O films were deposited by Radio-Frequency magnetron sputtering. The structural, optical and electrical properties were studied. Along with the increase of nitrogen flow rate, the preferred orientation of Cu2O films varied from (111) plane to (200) plane. In addition, may be due to the inhibition of oxygen vacancy by using Cu2O target, an obvious optical band gap narrowing was observed. Hall results indicated that the resistivity decreased sharply with the increase of carrier concentration. And the lowest value 2.1 Ωcm was obtained when the nitrogen flow rate was increased to 20 sccm. However, due to the defects which induced by N doping, the hall mobility is decreased slightly. X-ray photoelectron spectra results further demonstrated the formation of Cu2O films without other phases.

Defect-related optical bandgap narrowing and visible photoluminescence of hydrothermal-derived SnO2 nanoparticles

Tin oxide (SnO2) nanoparticles are fabricated by a novel hydrothermal route using H2O2 as oxidizer without adding any corrosive acids or bases. The morphology, composition, and microstructure are characterized by transmission electron microscopy, X-ray diffraction (XRD) and Raman spectra. XRD analysis reveals the formation of single phase rutile type tetragonal structure of all samples which is further supported by Raman studies. The average crystallite size is observed to vary from 2.8 to 4.2 nm as the reacting temperature increases from 120 to 180 °C, suggesting the promotion of crystal growth with increasing temperature. Raman spectroscopy measurement presents the existence of the defect modes, of which the mode at about 565 cm−1 is identified to relate to oxygen vacancies as this mode becomes more prominent in the as-grown sample. Unusual optical band gap narrowing and the excitation wavelength dependent visible luminescence are observed, and both are supposed to be induced by the intrinsic defects. The photoluminescence (PL) spectra of all samples consist of two defect-related subbands, i.e., orange and blue luminescence bands. The two subbands are attributed to optical transitions in oxygen vacancies and some intrinsic surface states, respectively. Spectral analysis suggests that the excitation wavelength dependences of the two subbands are due to a distribution of the band gap of the nanoparticles induced by the internal strain, defect concentration as well as the quantum confinement effect. This work provides a feasible route to modulate the band gap and defect emissions and a good understanding of the PL behavior in the present SnO2 nanoparticles.

Structural, Raman and photoluminescence properties of Fe doped WO3 nanoplates with anti cancer and visible light driven photocatalytic activities

Doping is an effective way to tailor the properties of metal oxides for various applications. Herein, a series of Fe doped WO3 nanoplates have been synthesized using a facile co-precipitation method. The successful inclusion of Fe ions doping in the monoclinic structure of WO3 has been confirmed by XRD, EDX, FTIR, XPS and Raman spectroscopy. The optical band gap of WO3 has been facilely tuned with Fe doping which is attributed to positive shift in valance band. The photoluminescence (PL) spectroscopy studies revealed the presence of large number of defects in the crystal structure of WO3. Interestingly, it has been observed that photocatalytic activity of WO3 nanoplates could be significantly enhanced with Fe doping. This enhancement has been linked to the narrowing of optical band gap and lower electron-hole pair recombination rate due to the trapping of electrons by the crystal defects. This study provides a facile and effective method to prepared Fe doped WO3 nanoplates with enhanced visible light driven photocatalytic degradation of methyl red. Furthermore, the Fe doped WO3 nanoplates have also shown highly efficient anticancer activities against human liver (Hep-2) and breast (MCF-7) cancer cells. The percent cell viability of both cancer cells lines is significantly decreased (up to 50%) with Fe doping.

Optical characterisation and device application studies of quaterthiophene (QT) thin films

Investigations on Optical characterization and device application of vacuum evaporated QT thin films have been reported. Narrowing of optical band gap has been reported for vacuum annealed QT thin films with increase in annealing temperatures. Switching characteristics of QT/C60 bilayer device on glass substrates indicated its use in write once read many (WORM) memory applications.

Structural and optical properties of (Zn, Co) co-doped SnO2 nano particles

Optical bandgap narrowing is observed in Sn0.94 Zn0.05Co0.01O2, Sn0.92 Zn0.05Co0.03O2 and Sn0.90 Zn0.05Co0.05O2 nanoparticles synthesized by the chemical co- precipitation method. The estimated particle size and optical band gap of Sn0.94 Zn0.05Co0.01O2, Sn0.92 Zn0.05Co0.03O2 and Sn0.90 Zn0.05Co0.05O2 nanoparticles are 16.51, 11.75 and 6.6 nm and 1.45, 1.36 and 0.93 eV, respectively. A red shift is noticed for all samples as compared to band gap value (3.6 eV) for bulk SnO2. Increasing Co content results in decreased particle size, narrowed band gap and enhanced emission intensities in visible range of light. Small sized nano particles have larger surface areas and these large surface areas lead to more defects (oxygen vacancies, tin interstitials). The increasing defects concentration (especially oxygen vacancies, tin interstitials) results to narrow the band gap. With increasing Co content, the particles are in oxygen-poor state indicating the presence of many O vacancies and Sn interstitials. The oxygen content decreases steadily with increasing Co content, with the Sn:O atomic ratio increases suggesting the high density of defects. Large number of defects (oxygen vacancies) creates several donor levels within the forbidden energy gap to narrow the band gap and these defects in the band gap act as luminescent centers to enhance the emission intensities in visible spectrum.

Sputtered cuprous oxide thin films and nitrogen doping by ion implantation

The structural, optical and electrical properties of sputtered cuprous oxide thin films have been optimized through post-deposition thermal treatments. Moreover we have studied the effects of nitrogen doping introduced by ion implantation followed by the optimized oxidant thermal annealing. Three concentrations have been used, 0.6N%, 1.2N%, and 2.5N%. Along with the preservation of the Cu2O phase, a slight optical band gap narrowing and a significant conductivity enhancement has been observed with respect to the undoped samples. These results can be justified by the absence of further oxygen vacancies promoted by dopant introduction and by the substitution of O atoms by N ones. This lattice configuration has been guaranteed by the post implantation annealing in oxidant atmosphere. The used doping technique represents an original out-of-equilibrium approach toward the formation of low-resistivity contacts on Cu2O films for photovoltaic applications.

Optical band-gap narrowing in perovskite ferroelectric ABO3 ceramics (A=Pb, Ba; B=Ti) by ion substitution technique

Band-gap narrowing effects have been observed in and Ni-doped PbTiO3 and Ni-doped Ba(Ti0.875Ce0.125)O3 ferroelectric ceramics. Structural characterizations and microstructural analysis show that the as-prepared PbTi0.67Ni0.33O3−δ (PTN) and Ba(Ti0.75Ce0.125Ni0.125)O3−δ (BTCN) samples exhibit polycrystalline perovskite structures with dense microstructures. The Ni doping at Ti-site results in lattice expansion and introduces oxygen vacancies. Note that the band-gap values of PTN and BTCN are 2.42eV and 2.73eV, respectively, much smaller than that of corresponding parent PbTiO3 (3.21eV) and BaTiO3 (3.37eV) materials. This optical behavior can be due to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. With such decreased optical band-gaps, the PTN and BTCN materials will have great potential in solar-energy conversion devices.

Niobium pentoxide (Nb2O5) thin films: rf Power and substrate temperature induced changes in physical properties

In the present work, thin Nb2O5 films were grown on glass substrates using rf magnetron sputtering technique, in a pure argon atmosphere at different rf power and various substrate temperatures. X-ray diffraction result revealed the amorphous nature of deposited films. The maximum optical transmittance of about 92% with clear interference pattern was realized from the optical study. A systematic reduction in energy band gap values with increasing substrate temperature and rf power could be attributed to the localized state near the band edges. This hypothesis is consistent with our photoluminescence study where we observe the slight shift in emission peak towards higher wavelength region due to the effect of optical band gap narrowing. The X-ray photoelectron spectroscopic study confirms the film purity and the presence of five-valent niobium (Nb5+) in Nb2O5 films. Surface morphological studies revealed the formation of Nb2O5 films with smooth, uniform and pinhole free in nature. Raman spectra show the NbO stretching mode of the deposited films.

The photovoltaic spectral response regulated by band gap in Zr doped Bi4Ti3O12 thin films

The pure and Zr doped Bi4Ti3O12 thin films have been prepared via the solution–gelation technique. The phase structure, optical band gap, and photovoltaic spectral response were investigated in detail. It shows that the photovoltaic responses of the short-circuit current for Zr doped Bi4Ti3O12 thin films have shown a noteworthy red-shift behavior compared with the pure Bi4Ti3O12 thin films. The halfway points of the full width at half-maximum are detected at 469 and 514 nm, closely corresponding to the measured band gap for the thin films. Thus photovoltaic response behavior is attributed to octahedral distortion and optical band gap narrowing. The present work provides an available way on controlling photovoltaic properties of Bi4Ti3O12 based thin films and accelerating its application in light sensors, light drivers and ferroelectric photovoltaic cells.

Fermi level stabilization and band edge energies in CdxZn1−xO alloys

We have measured the band edge energies of CdxZn1−xO thin films as a function of composition by three independent techniques: we determine the Fermi level stabilization energy by pinning the Fermi level with ion irradiation, measure the binding energy of valence band states and core levels by X-ray photoelectron spectroscopy, and probe shifts in the conduction band and valence band density of states using soft X-ray absorption and emission spectroscopy, respectively. The three techniques find consensus in explaining the origin of compositional trends in the optical-bandgap narrowing upon Cd incorporation in wurtzite ZnO and widening upon Zn incorporation in rocksalt CdO. The conduction band minimum is found to be stationary for both wurtzite and rocksalt alloys, and a significant upward rise of the valence band maximum accounts for the majority of these observed bandgap changes. Given these band alignments, alloy disorder scattering is found to play a negligible role in decreasing the electron mobility for all alloys. These band alignment details, combined with the unique optical and electrical properties of the two phase regimes, make CdZnO alloys attractive candidates for photoelectrochemical water splitting applications.

Spatial Location Engineering of Oxygen Vacancies for Optimized Photocatalytic H2 Evolution Activity

Enhanced H2 evolution efficiency is achieved via manipulating the spatial location of oxygen vacancies in niobates. The ultrathin K4 Nb6O17 nanosheets which are rich in surface oxygen vacancies show enhanced optical absorption and band gap narrowing. Meanwhile, the fast charge separation effectively reduces the probability of hole-electron recombination, enabling 20 times hydrogen evolution rate compared with the defect-free bulk counterpart.

Defect induced optical bandgap narrowing in undoped SnO2 nanocrystals

Unusual optical bandgap narrowing is observed in undoped SnO2 nanoparticles synthesized by the solution combustion method. The estimated crystallite size is nearly 7 nm. Though the quantum confinement effect predicts a larger optical bandgap for materials with small crystallite size than the bulk, the optical bandgap in the as synthesized materials is found to be 2.9 eV compared to the reported value of 3.6 eV for bulk SnO2 particles. The yellow-green photoluminescence emissions and the observed narrowing of the bandgap can be attributed to the deep donor levels of oxygen vacancies, owing to the high exothermicity of the combustion reaction and the faster cooling rates involved in the process.

The analysis of CdS thin film at the processes of manufacturing CdS/CdTe solar cells

In sequence, the deposited CdS thin film had undergone physical and optical changes by the processes of manufacturing CdS/CdTe solar cells. CdS thin film was manufactured by the Chemical Bath Deposition (CBD) method. The aqueous solution was based on ammonia solution. The temperature of bath system was 75 °C and deposition time was 50 min. The thickness of deposited CdS thin film was about 200 nm. The substrate was the glass coated with SnO 2:F thin film. The following process was the deposition of CdTe thin film by the Closed-Space-Sublimation (CSS) method. The final process was the CdCl 2 heat treatment at N 2+O 2 atmosphere, and the contrast experiment progressed for CdCl 2–CdS thin film after CSS process at N 2 atmosphere. The phase transition of CdS thin film, stress relaxation and optical band gap narrowing were developed by each process. And so, the formation of cadmium oxide was detected after the CdCl 2 heat treatment. It influenced to increase the optical band gap of CdS thin film. The variation in the structure properties, optical properties and residual stresses of CdS thin film was analyzed by X-ray diffractometer (XRD), Raman spectroscopy and ultraviolet (UV)–visible (VIS) spectroscopy.

Synthesis of C-Doped TiO2 Nanoparticles by Novel Sol-Gel Polycondensation of Resorcinol with Formaldehyde for Visible-Light Photocatalysis

Nanosized C-doped TiO 2 was prepared by a new method based on resorcinol-formaldehyde (RF) gel formation in situ during hydrolysis of Ti precursor (TTIP, titanium tetraisopropoxide). XRD, DRS, EDX, SEM and HRTEM results confirmed that the carbon in the sample existed as free carbon (C–C), carbonate carbon (–CO 3 ) and doped into lattice of TiO 2 (Ti–C), resulting in red shift of optical adsorption edge and band gap narrowing. The photo activity of the C-doped TiO 2 , tested on 2-chlorophenol, revealed that it was more active under visible light (k vis = 3.50 × 10 −3 min −1 ).

Influence of Ag, Cu dopants on the second and third harmonic response of ZnO films

Silver- and copper-doped ZnO films were prepared by radio-frequency (RF)-magnetron sputtering on glass and quartz substrates. The influence of dopants content on the microstructural evolution and optical as well as nonlinear optical (NLO) properties were investigated. It has been found that the grain sizes were enlarged with increasing of Ag, Cu dopants amount in ZnO films. The Ag or Cu doping leads to the optical band gap narrowing. Besides, the second-order NLO response of Ag- and Cu-doped ZnO films is lower than that of undoped ZnO film. The second harmonic generation (SHG) efficiency of the ZnO:Ag film was found to be higher than that of the ZnO:Cu film at the similar concentration of dopant. In addition, the decrease of the third harmonic generation (THG) response is observed in ZnO films with increasing of Ag or Cu dopant amount.

Band gap widening and narrowing in moderately and heavily doped n-ZnO films

In semiconductors, a widening of the optical band gap occurs because the lower states in the conduction band are blocked. At the same time band gap narrowing also occurs due to many body interactions on doping. This paper reports the analysis of widening and narrowing of optical band gap in sol–gel derived ZnO films moderately doped with Yttrium and heavily doped sputtered ZnO films with aluminum and scandium. The band gap was evaluated using optical transmission data. Carrier concentration was known from the Hall measurements. At high concentrations the effective change in band gap is found to be the difference of the band gap widening and band gap narrowing. At low concentration of dopant the many body theories do not apply and the experiments also show that the band gap narrowing is practically negligible at these concentrations and the effective band gap widening is determined by the band gap widening alone. The chemical nature of the dopant played practically no role.

Room-temperature deposition of nanocrystalline PbWO 4 thin films by pulsed laser ablation

Pulsed laser ablation (PLA) was applied to synthesize nanocrystalline PbWO4 thin films onto glass substrates. The effects of Ar background gas pressure on phase evolution, microstructures and optical characteristics of PbWO4 thin films were investigated in detail. The PLA processes were carried out at room temperature without substrate heating or post-annealing treatment. XRD and HR-TEM results revealed that the PbWO4 thin films are composed of nanocrystalline and amorphous phases. Moreover, the films contained a high density of lattice defects such as twin boundaries and edge dislocations. The crystallite size and crystallinity increased, which were associated with a change in surface morphology as the Ar pressure increased. Reduced tungsten states W5+ or W4+ induced by oxygen vacancies were observed at 10 Pa and the atomic concentration of all constituent element was almost stoichiometric, especially the [Pb]/[W] ratio, which was nearly unity above 50 Pa. The optical energy band-gap was 3.03 eV at 50 Pa and increased to 3.35 eV at 100 Pa, which are narrower than the reported value (4.20 eV). This optical band-gap narrowing could be attributed to localized band-tail states and new energy levels induced by the amorphous structure and inherent lattice defects.

Studies of carbon ion self-implantation into hydrogenated amorphous carbon films

Abstract The properties of polymer-like amorphous hydrogenated carbon thin films with low defect density have been studied. These films were implanted with carbon ions with a dose range of 1012–1016 cm−2. The purpose of the study is to investigate the effects of ion beam damage on this type of film. Optical absorption measurements observe a narrowing of the optical band gap, suggesting the introduction of a large number of defect states subsequent to the implantation resulting in the broadening of the band tails, only after a threshold ion dose of 1015 cm−2. Nuclear reaction analysis suggests also a reduction in the hydrogen content of the film which coincides with film thinning.

The Study on Microstructure by NMR, FTIR, Raman, Conductivity and Optical Bandgap in Hydrogenated Amorphous Silicon Prepared by Novel Fabrication Methods

ABSTRACTIt was found that hydrogen dilution in the SiH4/H2 mixture tend to show a sharp line-shape in the NMR spectra as the substrate temperature is higher than 300 °C. The hydrogen-atom-treatment method also produces the same effect at a lower substrate temperature about 250°C. The Raman scattering spectra show that the hydrogen-atom-treatment creates the microcrystalline structure at a temperature higher than 250°C while hydrogen dilution produces mixed phases containing amorphous phase and a small quantity of micro-crystalline phase. Together with the optical bandgap narrowing, the increase of the dark conductivity and the reduction of photo-to-dark conductivity ratio, these samples indicate that with more hydrogen incorporation during deposition and plasma hydrogen treatment, these films possess a much compact structure, and the degree of crystallinity of hydrogenated silicon film was found to be systematically changed.

Control of a-Si:H Film Properties by Phoyo-Assisted Plasma CVD

ABSTRACTCO2 laser irradiation onto growing a-Si:H surface caused a decrease of the optical bandgap down to 1.63 eV and a significant increase of the photosensivity. Primary effect of the laser irradiation was a conventional substrate heating, while the optical bandgap narrowing could not be explained by heating effect alone.